The present invention relates to a shaft for a collarless hip joint prosthesis.
Such a shaft for a collarless hip joint prosthesis can be inserted into the femur and there anchored without the use of cement.
Since certain drawbacks arise in connection with the use of bone cement, such as, for example the connection coming loose, efforts have been made to fasten endoprostheses without the use of bone cement, if possible. However, this also involves a number of difficulties. For example, if the portion of the femur which serves to receive the joint head as "collar", the introduction of force into the bone takes place only in the region of the projecting "collar" portion. But since, under such conditions, the forces exerted under load are relatively great in a narrowly defined region, the resulting "wiggling movement" may cause the prosthesis shaft to come loose.
Furthermore, collarless prostheses have become known which are tapered and wedge themselves into the drilled-out marrow chamber of the femur. Such a prosthesis is disclosed, for example, in European Pat. No. A3-0 027,159. These prostheses have the drawback that the wedge effect caused in the distal portion by the tapered prosthesis shaft is very great so that the radial stress on the bone under load is considerable and, particularly in an osteoporotic bone, there exists the danger of crack formation.
Due to the complicated configuration of the widening regions in the extreme proximal region of the shaft, it is very difficult, requiring precise measurements and normal anatomic conditions, to adapt the prosthesis to the marrow chamber in such a way that a defined force distribution results when the shaft is inserted. If the prosthesis is load bearing exclusively in this region, there again will be more or less localized stresses with the corresponding adverse results for the bone substance.
European Pat. No. A1-0 044,915 discloses a shaft portion for a collarless hip endoprosthesis which has an elliptical cross section, with the wedge fit taking place in the region of the bend or curvature of the prosthesis shaft subsequent to the Adam curve, so that, with the prosthesis under load, relatively small bending moments become effective and excessive stresses on the bone tissue are stated to be impossible. However, this prosthesis as well has the drawback that adapating the marrow chamber to the relatively complicated configuration can be accomplished only with difficulty. Moreover, the opening in the bone must be large so that the prosthesis can be inserted; this, particularly in its region having the greater curvature and the elliptical cross section, requires a larger opening than would correspond to the final cross section of the prosthesis near the joint head.